INFLUENCE OF ELEVATION ON SOUNDS. 217 



great violence and instantly fell down dead, 

 throwing up blood from his mouth and nose. 



Sounds of known character and intensity are 

 often singularly changed even at the surface of 

 the earth, according to the state of the ground 

 and the conditions of the clouds. On the 

 extended heath, where there are no solid objects 

 capable of reflecting or modifying sound, the 

 sportsman must frequently have noticed the un- 

 accountable variety of sounds which are produced 

 by the report of his fowling-piece. Sometimes 

 they are flat and prolonged, at other times short 

 and sharp, and sometimes the noise is so strange 

 that it is referred to some mistake in the loading 

 of the gun. These variations, however, arise 

 entirely from the state of the air, and from the 

 nature and proximity of the superjacent clouds. 

 In pure air of uniform density the sound is sharp 

 and soon over, as the undulations of the air 

 advance without any interrupting obstacles. In 

 a foggy atmosphere, or where the vapours pro- 

 duced by heat are seen dancing as it were in the air, 

 the sound is dull and prolonged ; and when these 

 clouds are immediately over-head, a succession of 

 echoes from them produces a continued or 

 reverberating sound. When the French astro- 

 nomers were determining the velocity of sound 

 by firing great guns, they observed that the report 

 was always single and sharp under a perfectly 

 clear sky, but indistinct, and attended by a long- 

 continued roll like thunder, when a cloud covered 

 a considerable part of the horizon. It is no 

 doubt owing to the same cause, namely, the 

 reflexion from the clouds, that the thunder rolls 

 through the heavens, as if it were produced by a 

 succession of electric explosions. 



